Phage display is a well-known technique for constructing and screening antibody libraries, whereby the protein of interest is expressed as a polypeptide fusion to a bacteriophage coat protein and subsequently screened by binding to immobilized or soluble biotinylated ligand. Traditional phage display, however, has several shortcomings. For example, some eukaryotic secreted proteins and cell surface proteins require post-translational modifications such as glycosylation or extensive disulfide isomerization, which are unavailable in bacterial cells.
Current yeast surface antibody display systems, such as cold capture, also suffer from various drawbacks. In the cold capture antibody display system, at low temperatures, the process of antibody release from host cell transport vesicles is delayed, so that the secreted antibody can be assayed on the cell surface for antigen binding. The cold capture method suffers from a low signal-to-noise ratio and identification of an antibody with specificity for the target antigen depends heavily on cellular expression levels of the antibody.
The affinity matrix system couples antibodies to the host cell surface, e.g., by biotin, where they can be assayed for antigen binding. The affinity matrix system exhibits a high incidence of cross-contamination between antibody clones. Antibodies may become decoupled from the host cell and, thus lose their link to the polynucleotides encoding their immunoglobulin chains.
Full length antibody display systems tether the full length antibody on the host cell surface by binding an immunoglobulin binding protein, such as protein A, that is fused to a cell surface anchor protein. The host cell contains polynucleotides encoding the antibody immunoglobulin chains. Typically, binding of the antibody occurs after the immunoglobulin binding protein is expressed on the cell surface. This system, thus, leads to some erroneous binding of the antibody to host cells that do not express the antibody. The present system and methods provide numerous advantages over these earlier methods.